The EF 100-400mm II was definitely NOT "made to pair with the 5D Mark IV". Sure, the two will work quite well together. But the lens will also work fine on any other Canon DSLR, including your SL1... It, or any other relatively large lens, will just seem a bit unbalanced due to the small size and light weight of your camera. In fact, a lot of "large lens" users like to use a battery grip no their cameras with a second battery, to add some mass to the body and better balance a hefty lens. The SL1 is simply the smallest, lightest DSLR anyone has made... and that's not possible with the SL1, which doesn't have any means of adding a battery grip.

Not that the 100-400s are all that heavy. The original push/pull zoom model weighs just over 3 lb., while the "II" gained some weight to about 3.5 lb. To put it in perspective, it's about the largest lens I'd want to shoot with hand held for a few hours. Any longer than that, I put it on a tripod. The new Sigma 100-400mm is a little bit smaller and under 3 lb. But it's also 2/3 stop slower than the Canon lenses throughout most of it's zoom range, and 1/3 stop slower the rest. The Siggy also doesn't include a tripod mounting ring or have any means of adding one (both the Canon lenses have tripod rings, which are included in the weights). There also is the new Tamron 100-400mm, slightly larger and heavier than the Siggy, but less than the Canon. The Tamron can optionally be fitted with a tripod ring (sold separately), but once again it's not included in the weight statistics (probably adds 4 or 5 oz.)

In fact, for sports and wildlife shooting, I'd recommend using the 100-400mm on one of the APS-C cameras instead of the full frame 5DIV. Either the 20MP 7D Mark II (which is essentially the same size and weight as the full frame camera) or the 24MP 80D work great for those purposes, with that lens and others. The APS-C cameras are like having a "free 1.6X teleconverter" on the lens... "free" meaning that there's no loss of light like there is with an actual teleconverter. When using a telephoto and to photograph smaller, more distant subjects, either of those APS-C models.... or any of the other 24MP APS-C models... or even your 18MP SL1 and other 18MP models will actually "put more pixels on target" than the full frame camera.

Another way of looking at this... if you were to use the same 400mm lens on both cameras to shoot the same subject from the same distance... then cropped the 30MP 5DIV's image down to APS-C dimensions, you'll be left with approx. 12MP image! Heck, your SL1 has 50% more resolution than that... while a 24MP SL2, T7i, 77D or 80D has a full 100% more. In order to take full advantage of all the 5DIV's "full frame goodness" and out-perform what the 100-400mm can do on an APS-C model, you'll need to fit a 500mm or 600mm lens to the full frame 5DIV.

I LOVE my 77D. The give-ups over the 80D are: 11 oz. lighter, a smaller battery (Rebel Style) and a Penta Mirror Rebel style). I can't see the difference thru the viewfinder (74 year old surgically enhanced and cataract lens replaced eyes), With 2 batteries, It outlast ME. Otherwise I think it is the same camera.
In reality if you're thinking weight is a priority, as I do, the thought of hand holding the 100-400 because of the weight AND the equivalent 640mm reach is a skills and body stability extreme for almost all of us.

BTW: The 70-300 II nano lens is excellent. See Ken Rockwell's review. While he extols the 100-400, he is quite complimentary on the 70-300. As am I. The 77D and 70-300 II IS is quite hand hold-able--even for me.

A couple of weeks ago, I posted a question about my Canon camera with the 70-300 lens and the lack of focus. I received lots of great suggestions and reassurance that the combination should provide sharp pictures. I have to admit that I accidently omitted a very important bit of information when I posted my initial question. Last summer when we were in Seattle, I was walking, carrying the camera around my neck when I fell flat on my face. I ended up with badly bruised ribs. And although I didn't realize it, the lens received a bent mounting ring . . . the source of all my problems. So much time lapsed between when I fell and when I used this lens again, I just forgot about it. Anyway, the local camera shop replaced it and it's working fine. I'm posting a couple of pictures taken since the repair. Unfortunately, I've now decided that what I need for the type of photography I want to do is probably a 100-400mm L IS USM lens. And even more unfortunately, from what I've read, my Rebel SL1 is probably too light to handle this lens comfortably. Any suggestions? Thanks again.

Sally, your question can be answered easily with: is the SL1 comfortable for you to use and are the functions you want to use, easily accessible? If the answer is yes then use that body on that lens. An advantage of moving up in a camera body might be more frames per second burst, a feature useful with wildlife photography. Another factor is the f/stop necessary for the auto focus to work. I often use a 1.4X tele-extender which means your f/5.6 lens now only lets in f/8 amount of light. Will your camera body auto-focus with an f/8 lens? Just a few things to consider ... but without using a tele-extender, I'd be a happy camper and the SL1.